(1) Field of the Invention
This invention relates to a method of fabrication used for semiconductor circuit devices, and more specifically to an improved method of forming the metallization interconnection layers in DRAM (Dynamic Random Access Memory) products.
(2) Description of Related Art
DRAM (Dynamic Random Access Memory) products make use of very low leakage associated with gate circuits and junctions of MOS devices. Usually the leakage currents are small enough to permit the circuit's parasitic capacitances to exhibit time constants of many milliseconds. And, long time constants may be used to provide temporary storage, which may be made permanent by appropriate cyling or refresh operations. It is, therefore, desirable that the refresh time (time between refresh cycles) be as long as possible in order to minimize the necessity to periodically restore charge storage in the memory cells. More advanced DRAM products require longer and longer refresh times to ensure quality and advanced memory functionality.
In current DRAM products, such as 16M DRAM, it is desirable that the mean refresh time be greater than 1200 milliseconds. However, using current fabrication processing technologies, the typical refresh time is about 400-600 milliseconds. This results in refresh time test failure and becomes a major yield detractor when manufacturing advanced DRAM products such as 16M DRAM, 64M DRAM and 256M DRAM. Improvements to DRAM manufacturing processes which can increase the refresh time are, therefore, desirable. Such process improvements should, also, not adversely impact overall process yield and cost.
U.S. Pat. No. 5,420,070 entitled "Manufacturing Method of Interconnection Structure of Semiconductor Device" granted May 30, 1995 to Megumi Matsuura et al describes a semiconductor manufacturing process in which a thermal treatment is made to a tungsten-titanium interface to cause reaction of the tungsten and the titanium layers. The result is an improved metallization structure having stable resistance at contacts and high process yield.